Liquid precursor for deposition of copper selenide and method of preparing the same

ABSTRACT

Liquid precursors containing copper and selenium suitable for deposition on a substrate to form thin films suitable for semiconductor applications are disclosed. Methods of preparing such liquid precursors and methods of depositing a precursor on a substrate are also disclosed.

RELATED APPLICATION

This application claims the benefit of priority from U.S. ProvisionalPatent Application Ser. No. 61/396,053, entitled “Liquid Precursor forDeposition of Copper Selenide and Method of Preparing the Same,” filedMay 21, 2010.

CONTRACTUAL ORIGIN

The United States Government has rights in this invention under ContractNo. DE-AC36-08GO28308 between the United States Department of Energy andthe Alliance for Sustainable Energy, LLC, the Manager and Operator ofthe National Renewable Energy Laboratory.

BACKGROUND

Compounds of Groups IB, IIIA and VIA, especially copper indiumdiselenide (CIS) and copper indium gallium diselenide (CIGS), have beenstudied as semiconductor materials for a number of thin-filmsemiconductor applications. One key application is their use as lightabsorbing materials in solar cell components. The elements which formthese compounds are relatively common and fairly inexpensive, and whenformulated and processed into light absorbing materials (e.g., CIS andCIGS), they are highly efficient in converting solar energy toelectrical energy.

Unfortunately, cost effective methods of fabricating these lightabsorbing materials, especially in the form of thin films have beenelusive and limited at best. Most current fabrication methods of lightabsorbing materials (e.g., CIS and CIGS) rely on vacuum depositiontechniques (e.g., physical vapor deposition), which are generallyexpensive and labor-intensive.

Recent advances in the thin film technology involve the use of liquidprecursors to deposit precursors of light absorbing materials. Liquidprecursors for use in thin film deposition represent less expensivealternatives to vacuum deposition technology. Liquid precursors providedistinct advantages over conventional vacuum deposition technologyincluding higher throughput, lower cost and more efficient materialutilization. In addition, liquid precursors are compatible with abroader range of substrate types and surface morphologies including verylarge substrates or those having considerable flexibility.

Liquid precursors are generally formulated to contain a combination ofmetal and a multinary chalcogenide material each selected, respectively,from the elements of Group IB, Group IIIA and Group VIA, utilizinghydrazine as a solvent. Upon deposition, the liquid precursor convertsinto a desired solid precursor or a metal chalcogenide through theapplication of heat. The deposited solid precursor can then be processedvia suitable means in combination with other solid precursors to producethe final light absorbing material (e.g., CIS and CIGS).

The use of hydrazine as a solvent is problematic. Hydrazine is avolatile, corrosive liquid that is expensive, highly toxic anddangerously unstable. Its use therefore is strictly controlled. For thesame reasons, hydrazine-containing liquid precursors require specialcare and handling, and implementation of extensive safety measures.Thus, the cost and difficulty associated with making and usinghydrazine-containing liquid precursors is considerably high.

SUMMARY

The following embodiments and aspects thereof are described andillustrated in conjunction with systems, tools and methods that aremeant to be exemplary and illustrative, not limiting in scope. Invarious embodiments, one or more of the above-described problems havebeen reduced or eliminated, while other embodiments are directed toother improvements.

An exemplary method of preparing a liquid precursor is disclosed havinga copper selenide content defined by the formula Cu_(x)Se_(y) wherein xand y are each in the range of 1 to 2 wherein x+y is in the range of 2to 3. Such liquid precursors are suitable for forming a solid precursoron a substrate, for example, in the form of thin films, which may beused, for example, in semiconductor applications such as the preparationof light absorbing materials for solar cells. The solid precursor isgenerally formed by heating the liquid precursor to a temperature andfor a time sufficient to drive off the liquid components.

The exemplary method produces a liquid precursor in the form of a liquidbased material or composition that does not contain hydrazine and can beused in deposition techniques that are easier, more efficient and morecost effective to use than solid based deposition techniques such asvacuum deposition. The exemplary liquid precursors allow for depositionby suitable deposition techniques such as drop coating, dip coating,spin coating, spraying, brushing, air brushing, ink jet application,stamping, printing, pouring, wiping, smearing, spray deposition, slotcoating, and other methods of applying liquids to the surface of asubstrate. For example, the deposition technique may be spray depositionor slot coating.

The exemplary method eliminates the use of hydrazine as a solvent, thuseliminating all procedures known to be used in handling and removinghydrazine. The resulting liquid precursor is essentially hydrazine-free,thereby greatly enhancing safety and reducing costs of the process offorming the thin films. The exemplary hydrazine-free liquid precursorspermit deposition of solid precursors in a safer and more cost effectivemanner than those, which contain hydrazine. In addition, the exemplarymethod produces liquid precursors with higher precursor (i.e.,copper-selenide) concentration levels, thus reducing the time necessaryfor generating the solid precursor. The exemplary liquid precursors canbe used to form thin films having a desirable copper selenidecomposition suitable for use in forming CIS or CIGS thin films useful inthe fabrication of solar cells.

Accordingly, an exemplary embodiment is directed to a method ofpreparing a liquid precursor which includes:

reducing elemental selenium with a stoichiometric amount of anitrogen-containing reducing agent in the presence of a first solvent toyield a preliminary precursor solution; and

combining the preliminary precursor solution with a solution of a coppersalt and a second solvent, which may be the same or different than thefirst solvent, to yield the liquid precursor.

Another exemplary embodiment is directed to a method of depositing asolid precursor on a substrate, which includes:

applying a liquid precursor prepared by the exemplary method describedabove to the substrate; and

heating the liquid precursor to a temperature and for a time sufficientto yield the deposited solid precursor on the substrate.

In another exemplary embodiment, there is provided a liquid precursorwhich includes:

a hydrazine-free solvent for a solute comprising copper and selenium;and

a solute comprising copper and selenium.

In addition to the exemplary aspects and embodiments described above,further aspects and embodiments will become apparent by reference to thedrawings and by study of the following descriptions.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments are illustrated in referenced figures of thedrawings. It is intended that the embodiments and figures disclosedherein are to be considered illustrative rather then limiting.

FIG. 1 is a schematic view of chemical reaction steps of an exemplaryembodiment of a method for preparing a liquid precursor;

FIG. 2 is a trace view of atomic percent of copper and selenium as afunction of processing temperature for a given time, representingexemplary embodiments;

FIG. 3 is a trace view of an X-ray diffraction pattern representing anexemplary embodiment of a Cu—Se film dropcast on glass at 200° C.;

FIG. 4 is a trace view of an X-ray diffraction pattern representing anexemplary embodiment of a precursor film comprising layers of In—Ga—Seand Cu—Se supported on molybdenum and glass prior to thermal treatment;

FIG. 5 is a trace view of an X-ray diffraction pattern representing anexemplary embodiment of a CIGS film formed after thermal treatment ofthe precursor film of FIG. 4;

FIG. 6 is a trace view of a current-voltage (I-V) curve underillumination showing the performance characteristics of a photovoltaicdevice utilizing a CIGS film produced from a Cu—Se liquid precursorrepresenting an exemplary embodiment; and

FIG. 7 is a trace view of a current-voltage (I-V) curve underillumination showing the performance characteristics of a photovoltaicdevice utilizing a CIGS film produced from a Cu—Se precursor depositedthrough physical vapor deposition (PVD).

DETAILED DESCRIPTION

An exemplary liquid precursor and method of preparing the same isdisclosed which is suitable for depositing a desired chemical species orprecursor (i.e., copper selenide) on a substrate. The deposited desiredchemical species can then be heated to remove volatile componentsincluding solvent to yield a solid precursor, for example, in the formof a thin film. The solid precursor of the desired chemical species canbe used in forming a CIS (copper-indium-selenide) and/or CIGS(copper-indium/gallium-diselenide) light absorbing material for solarcells.

The liquid precursor of one exemplary embodiment does not employhydrazine as a solvent. Accordingly, there is no hydrazine present inthe liquid precursor and therefore special efforts to handle and removehydrazine are eliminated. The exemplary liquid precursor comprises amolar ratio of Cu:Se of about 1:3. Typically, most of the selenium isassociated with the copper while a minor portion of selenium will bepresent in elemental form. The liquid precursor exhibits a relativelyhigh concentration level of copper and selenium suitable for rapidlydepositing a thin film on a substrate. The copper concentration in thisliquid precursor is in the range of from about 0.08 M to about 0.10 M,whereas prior art liquid precursors with hydrazine as a solventtypically have a copper concentration of from 0.02 M to 0.04 M.

Copper selenide-containing thin films are useful in the fabrication ofCIS and/or CIGS light absorbing materials for solar cells. The copperselenide layer and the indium and/or gallium selenide layer(s) areplaced into contact under reactive conditions including heat to form adesirable light absorbing material. An exemplary form of copper selenidefor the light absorbing material is CuSe and/or CuSe₂. During heating toform the light absorbing material the amount and duration of heat can betailored to control the molar ratio of Cu:Se. One such example isdisclosed in U.S. patent application Ser. No. 12/658,204, filed Feb. 4,2010, incorporated herein by reference. Still other examples are alsocontemplated.

In an exemplary embodiment, there is provided a method of preparing aliquid precursor composition having a desirable copper selenide content.The liquid precursor can be applied to a substrate such as glass andsimultaneously thermally treated in a manner which provides a solidprecursor, for example, in the form of a thin film, having a targetcopper selenide content as described above. The Cu:Se ratio maydetermined by any suitable chemical analysis technique such as, forexample, inductively coupled plasma atomic emission spectroscopy(ICP-AES).

An exemplary method for preparing one exemplary embodiment of the liquidprecursor is represented in FIG. 1. The exemplary method involvesreducing elemental selenium with a stoichiometric amount ofnitrogen-containing reducing agent, such as hydrazine, in the presenceof a first solvent (excluding hydrazine) to yield a preliminaryprecursor solution, and combining the preliminary precursor solutionwith a solution of a copper salt and a second solvent (excludinghydrazine) to yield the liquid precursor as indicated by equations (1)and (2), respectively. Typically, any precipitate formed during thecombining step is separated from the liquid precursor as indicated byequation (3). The precipitate may be separated from the liquid precursorby any suitable separation technique including, but not limited to,filtration, and centrifugation.

The term “nitrogen-containing reducing agent” is intended to refer to achemical compound, typically containing nitrogen, which exhibits astandard reduction potential less than the standard reduction potentialof selenium, and which is consumed in the oxidation-reduction reactionwith selenium to yield by-products which do not adversely affect thereducing reaction and are relatively harmless from an environmentalstandpoint. An exemplary example of the nitrogen-containing reducingagent is hydrazine.

As discussed above, the nitrogen-containing reducing agent, for example,hydrazine, is used to reduce elemental selenium in the presence of afirst solvent to form the preliminary precursor solution as indicated inequation (1) of FIG. 1. The term “preliminary precursor solution” isintended to refer to the mixture of the reduced elemental selenium andthe first solvent prior to mixing with a solution of a copper salt and asecond solvent.

The copper salt may be selected from any soluble copper salts includingCu⁺² salts such as, for example, copper chloride, copper bromide, copperiodide, copper acetate, copper formate, copper nitrate, coppertrifluoromethanesulfonate, and the like. The first solvent is anysolvent which facilitates the reduction reaction of selenium (seeEquation 1 of FIG. 1). The second solvent is any solvent whichfacilitates the reaction shown in Equation 2 of FIG. 1. Examples of thefirst and second solvents include, but are not limited to, aminesincluding primary and secondary amines, and glycols. Further examples ofthe first and second solvents include, but are not limited to, ethylenediamine, pyridine, ethanolamine, diethylene triamine, and ethyleneglycol. The first and second solvents may be the same or different.

The nitrogen-containing reducing agent (e.g., hydrazine) is reacted withelemental selenium in stoichiometric amounts. In the exemplary method,utilizing hydrazine as a reducing agent (not as a solvent) in astoichiometric amount ensures that hydrazine is completely consumed inthe reaction with elemental selenium yielding nitrogen gas.

The preliminary precursor solution is thereafter combined or blendedwith the solution of the copper salt and the second solvent to yield theliquid precursor as indicated in equation (2) of FIG. 1. As indicated inequation (3) of FIG. 1, any precipitate formed from the reaction of thepreliminary precursor solution and the copper salt may be separated fromthe resulting liquid precursor using conventional separation techniques(e.g., filtration, centrifugation).

In an exemplary method of depositing a solid precursor on a substrate,the resulting exemplary liquid precursor is applied to the substrateunder elevated temperature conditions for a time sufficient to removethe solvent and other volatile components. During this thermal processstep, the exemplary liquid precursor converts into a solid precursor(i.e., Cu—Se), for example, in the form of a thin film. The selection ofa temperature and duration of heating have been determined to controlthe atomic ratio of copper to selenium when the precursor composition isdeposited on the substrate (i.e., the relative amount of Cu and Se inthe thin film) as shown in FIG. 2. Relatively low temperatures favor theformation of a selenium rich species (CuSe₂). Relatively highertemperatures favor the formation of the copper rich species (Cu₂Se).Thus, raising the reaction temperature tends to raise the copper contentand lower the selenium content.

For example, deposition of the copper selenide liquid precursor at atemperature of from about 50° C. to 275° C., where about 200° C. favorsformation of CuSe₂. If deposition is conducted at about 275° C. to 350°C., e.g., about 325° C., the predominant species is CuSe. Astemperatures rise above about 350° C., the copper selenide liquidprecursor gradually favors the formation of undesirable Cu₂Se.Accordingly, by controlling the temperature of the deposition processwithin the temperature range described above, the content of the copperselenide compounds can be precisely controlled.

In the formation of CIS and CIGS absorption layers, copper selenidelayers containing substantially pure CuSe may be used. Accordingly, anexemplary method of forming a CIS or CIGS absorption layer is to depositthe copper selenide layer at a temperature from about 100° C. to 350°C., for example, about 325° C.

The exemplary liquid precursors allow for deposition by suitabledeposition techniques such as drop coating, dip coating, spin coating,spraying, brushing, air brushing, ink jet application, stamping,printing, pouring, wiping, smearing, spray deposition, slot coating, andother methods of applying liquids to the surface of a substrate. Forexample, the deposition technique may be spray deposition or slotcoating.

In an exemplary embodiment, the liquid precursor can be deposited in asingle step heat treating method without resorting to multiple stepprocesses in which the last heating step is rapid thermal processing(RTP). In particular, the liquid precursor may be heated and converteddirectly to the desirable copper selenide species as the liquidprecursor is deposited on the substrate.

Rapid thermal processing (RTP) is defined herein as a heating regimen inwhich the target film is heated to a desired temperature in a shorttime, e.g., no more than about 10 minutes. The desired temperature ismaintained until the heating process is completed.

In a further exemplary method of depositing a solid precursor on asubstrate, the exemplary liquid precursor may be deposited on thesubstrate to form a solid precursor in the form of a thin film.Thereafter, the deposited liquid precursor is annealed at elevatedtemperatures to yield a copper selenide film containing CuSe₂ as thepredominant species. In the exemplary method, heating may be conductedwhile the exemplary liquid precursor is being deposited on the substratein a single step process.

It will be understood that the one step heating process is exemplary andnot required. Thus, the liquid precursor described herein may beinitially deposited on a substrate at relatively low temperatures andthereafter treated at higher temperatures including rapid thermalprocessing.

The Cu—Se containing liquid precursor representing an embodiment makesefficient use of selenium and in an exemplary embodiment obviates theneed for multiple heating steps. Because Cu—Se is produced in arelatively pure form, the liquid precursors can be used effectively tofacilitate the formation of, for example, CIS or CIGS with large crystalgrains in a solid state reaction with In—Se and optional Ga—Se.

In reference to FIG. 3, an, exemplary embodiment of the present copperselenide liquid precursor was dropcast on a glass substrate at about200° C. and the resulting film was characterized by X-ray diffraction.The X-ray diffraction pattern of the film deposited at about 200° C.shows a number of peaks identifying crystalline phases of CuSe₂ and Se.

In reference to FIG. 4, an exemplary embodiment of the present copperselenide liquid precursor was spray deposited at about 100° C. on alayer of indium, gallium and selenium to yield a stacked precursor filmof In—Ga—Se and Cu—Se. The stacked precursor film overlays a layer ofmolybdenum supported on a glass substrate. The stacked precursor film ischaracterized by the X-ray diffraction pattern as shown in FIG. 4. Thestacked precursor film was heat treated or annealed to convert the filminto a CIGS light absorbing material. In reference to FIG. 5, the X-raydiffraction pattern confirms the formation of the CIGS light absorbingmaterial from the stacked precursor film.

In reference to FIG. 6, the CIGS light absorbing material wasincorporated into a solar cell device through the addition of layers ofcadmium sulfide and zinc oxide and metal contacts. The solar deviceswere tested using a solar simulator under 1.5 AMU illumination. Contactprobes were placed on the front metal contacts and the back contact ofmolybdenum. The current was measured while performing a voltage sweepvia the contact probes. From the resulting data, the amount of appliedvoltage required to stop the current flow, or open circuit voltage(V_(oc)), the current flow when no voltage is applied, or short circuitcurrent, were determined. The short circuit current density (J_(SC)) wascalculated from the measured short circuit current and the surface areaof the device. The device efficiency is related to the fill factor (FF),which is determined from the sharpness of the device curve where a rightangle indicates 100% FF. The device efficiency can be determined fromthe product of the values of the V_(OC), J_(SC) and FF.

The data as shown in FIG. 6 indicates the performance characteristics ofa solar cell device fabricated with the present Cu—Se liquid precursor.The data as shown in FIG. 7 indicates the performance characteristics ofa solar cell device fabricated with Cu—Se precursor deposited viaphysical vapor deposition. A comparison of the performancecharacteristics of FIGS. 6 and 7 show that the solar cell devicefabricated with the present Cu—Se liquid precursor exhibited betterefficiency than the PVD Cu—Se-based solar cell device.

EXAMPLES

Specific embodiments will now be further described by the following,nonlimiting examples which will serve to illustrate in some detailvarious features. The following examples are included to facilitate anunderstanding of ways in which an embodiment may be practiced. It shouldbe appreciated that the examples, which follow represent embodimentsdiscovered to function well in practice. However, it should beappreciated that many changes can be made in the exemplary embodimentswhich are disclosed while still obtaining like or similar resultswithout departing from the spirit and scope of the claims. Accordingly,the examples should not be construed as limiting the scope of theclaims.

Example 1

A solution of selenium in ethylene diamine was prepared by placingselenium powder (2.38 g, 0.030 mole) and ethylene diamine (40 mL) in aflask under a nitrogen atmosphere and adding anhydrous hydrazine (0.160g, 0.005 mole). The resulting red solution was added over a 45 minperiod to a stirred solution of copper formate (1.54 g, 0.010 mole) in40 mL of pyridine to produce a dark red solution containing a smallamount of solid precipitate. The precipitate was removed by filtrationand the composition of the precursor solution was measured usingInductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES). Theprecursor composition found was 25.4 atomic % Cu and 74.6 atomic % Se(Se/Cu=2.94). The precursor solution was spray deposited on a glasssubstrate at 250° C. and the composition of the resulting film wasdetermined by X-ray fluorescence (XRF). The composition found was 33atomic % Cu and 66 atomic Se (Se/Cu=2.0).

Similar results were obtained when copper formate was dissolved inethylene diamine or ethanolamine in place of pyridine.

Example 2

A solution of copper nitrate (1.21 g, 0.005 mole) in ethylene diaminewas prepared by adding 20 mL of ethylene diamine slowly to the solid ina flask under nitrogen atmosphere. This was added to a solution ofselenium in ethylene diamine prepared as described in Example 1 from0.79 g Se (0.010 mole), 20 mL ethylene diamine and 0.080 g hydrazine(0.0025 mole), resulting in a dark red solution with a small amount ofprecipitate. The precipitate was removed by filtration and thecomposition of the precursor solution was measured using ICP-AES. Theprecursor composition found was 25.6 atomic % Cu and 74.4 atomic % Se(Se/Cu=2.91).

Example 3

A solution of selenium in ethylene diamine was prepared as described inExample 1 from 2.38 g Se (0.030 mole), 40 mL ethylene diamine and 0.160g hydrazine (0.005 mole) and added to a solution of copper chloride(1.70 g, 0.010 mole) in ethylene diamine (40 mL) and water (1 mL) over aperiod of 30 min. The solution turned dark red and a small amount ofprecipitate formed. The precipitate was removed by filtration.

Example 4

A solution of copper trifluoromethanesulfonate (1.10 g, 0.005 mole) inethylene diamine was prepared by adding 20 mL of ethylene diamine slowlyto the solid in a flask under nitrogen atmosphere. This was added to asolution of selenium in ethylene diamine prepared as described inExample 1 from 0.79 g Se (0.010 mole), 30 mL ethylene diamine and 0.080g hydrazine (0.0025 mole), resulting in a dark red solution with a smallamount of precipitate. The precipitate was removed by filtration.

While a number of exemplary aspects and embodiments have been discussedabove, those of skill in the art will recognize certain modifications,permutations, additions, and sub-combinations thereof. It is thereforenot intended that the following appended claims and claims hereafterintroduced and interpreted to include all such modifications,permutations, additions, and sub-combinations as are within their truespirit and scope.

1. A method of preparing a liquid precursor, said method comprising thesteps: reducing elemental selenium with a stoichiometric amount of anitrogen-containing reducing agent in the presence of a first solvent toyield a preliminary precursor solution; and combining the preliminaryprecursor solution with a solution of a copper salt and a secondsolvent, which may be the same or different than the first solvent, toyield the liquid precursor.
 2. The method of claim 1 wherein thenitrogen-containing reducing agent having a standard reduction potentialless than the standard reduction potential of selenium.
 3. The method ofclaim 1 wherein the nitrogen-containing reducing agent is hydrazine. 4.The method of claim 1 wherein the first and second solvents are eachselected from the group consisting of primary amines, secondary amines,glycols and combinations thereof.
 5. The method of claim 1 wherein thefirst and second solvents are each selected from the group consisting ofethylene diamine, pyridine, ethanolamine, diethylene triamine, ethyleneglycol, and combinations thereof.
 6. The method of claim 5 wherein thefirst and second solvents are ethylene diamine.
 7. The method of claim 1wherein the copper salt is selected from the group consisting of copperchloride, copper bromide, copper iodide, copper acetate, copper formate,copper nitrate, copper trifluoromethanesulfonate, and combinationsthereof.
 9. The method of claim 1 further comprising the step ofseparating any precipitate formed during the combining step.
 10. Themethod of claim 9 wherein the separation step is carried out using aprocess selected from the group consisting of filtration,centrifugation, and combinations thereof.
 11. The method of claim 1wherein the liquid precursor comprises a copper concentration of fromabout 0.08 M to 0.10 M.
 12. A method of depositing a solid precursor ona substrate, said method comprising the steps of: applying the liquidprecursor prepared by the method of claim 1 to the substrate; andheating the liquid precursor to a temperature and for a time sufficientto yield the deposited solid precursor on the substrate.
 13. The methodof claim 12 wherein the deposited solid precursor is a thin film. 14.The method of claim 12 wherein the temperature is at least 50° C. 15.The method of claim 14 wherein the temperature is from about 50° C. toabout 275° C.
 16. The method of claim 12 wherein the deposited solidprecursor is copper selenide at least substantially in the form ofCuSe₂.
 17. The method of claim 12 wherein the deposited solid precursorcomprises elemental selenium.
 18. The method of claim 12 wherein thedeposited solid precursor is characterized by an X-ray diffractionpattern substantially as shown in FIG.
 2. 19. The method of claim 12wherein the applying step is implemented through a process selected fromthe group consisting of drop coating, dip coating, spin coating,spraying, brushing, air brushing, ink jet application, stamping,printing, pouring, wiping, smearing, spray deposition, slot coating, andcombinations thereof.
 20. The method of claim 12 wherein the liquidprecursor comprises a copper concentration in the range of about 0.08 Mto about 0.10 M.
 21. A liquid precursor comprising: a hydrazine-freesolvent for a solute comprising copper and selenium; and a solutecomprising copper and selenium.
 22. The liquid precursor of claim 21wherein copper and selenium are present in a molar ratio of about 1:3.23. The liquid precursor of claim 21 wherein the hydrazine-free solventcomprises solvent selected from glycols, amines, and combinationsthereof.
 24. The liquid precursor of claim 21 wherein the copperconcentration is in the range of about 0.08 M to about 0.10 M.